Currently, in the field of security systems, motion detectors are generally provided to detect intruders. Dual technology motion detectors incorporate multiple sensing technologies in order to provide efficient intruder detection with minimized risk of false alarms. Many dual detectors incorporate passive infrared (PIR) technology and microwave (MW) technology.
PIR technology has long been used in motion detectors. The PIR sensor detects the difference between the infrared energy emitted from an intruder and that emitted from the ambient environment. Many PIR detectors utilize Fresnel lenses or custom shaped mirrors to focus infrared energy on a pyrodetector. The output signal from the pyrodetector is then processed via analog hardware and/or digital signal processing. Lenses and mirrors are designed to provide various detection zones emanating radially from the sensor. As a target moves across the PIR detection zones, the sensing elements within the pyrodetector are alternately exposed to the target IF energy, resulting in an alternating voltage output from the PIR sensor. The amplitude and frequency of this voltage vary with a number of factors including target size, speed, and direction relative to the PIR zones, difference between ambient and target temperature, width and spacing between the detection zones, and frequency response of the pyrodetector.
Upon receiving the signals, the detector may perform processing by comparing the received signal to one or more voltage thresholds. These threshold crossings produce positive and negative pulses that can be counted and timed, with certain combinations of pulse height, duration, and frequency being considered PIR alarms.
MW technology often operates on the principle of phase shift or Doppler effect. Unlike PIR, MW technology is an active technology. The MW detector transmits MW energy, which reflects off objects and returns to the MW detector. Moving objects result in a received signal that is frequency shifted from the original transmitted signal. The detector receives this signal, and generates an alternating voltage difference frequency signal which is then processed via hardware or digital signal processing. Because only the AC (alternating current) component of the signal is processed, only moving objects are detected. The frequency of the returned signal is dependent upon the target movement direction and speed. The amplitude is a function of transmitted signal strength, target size, distance, and reflectivity. Highly reflective materials such as metal will return significantly greater energy to the detector than less reflective objects such as people or animals. Processing may include comparison of the MW signal to one or more thresholds with certain combinations of quantity, duration, or frequency of threshold crossings considered MW alarms.
Typically, dual technology detectors incorporate an AND function, whereby both individual sensor technologies must be in the alarm state simultaneously to produce an alarm at the detector output. PIR and MW technologies have many differing and distinct potential false alarm sources, as well as some common sources such as small animals. Minimization of false alarms from either detector is likely to greatly reduce the incidence of false alarms for the dual detector system.
PIR motion detectors are susceptible to false alarms caused by small animals close to the detector. A small animal near the detector may provide similar infrared energy levels to the PIR sensor as a human at greater distances. In cold environments, the greater differential between small animal infrared emissions and that of the background can exacerbate this problem. Insects crawling on a detector cover or flying very near the detector also pose a threat in cases where the background includes any kind of stationary object with a temperature different from the ambient temperature, such as a heat source.
Some currently existing systems, such as that of U.S. Pat. No. 5,578,988 minimize PIR false alarms. This system includes a thermistor, which detects ambient temperature in order to adjust a PIR alarm threshold if necessary. However, this system does not consider a distance of the target object from the PIR detector. A solution is needed that minimizes PIR false alarm effects by considering target location in order to improve dual detector accuracy.